Project Title: A stable isotope method to rapidly screen the foraging ecotype profiles of Australian sea lion subpopulations: improving foraging distribution models to assist bycatch mitigation in gillnet fisheries
Chief Investigators: Associate professor Simon Goldsworthy, Professor Steve Donnellan & Associate Professor Rob Harcourt
Activity undertaken
Geospatial data sets (location and dive data) were collected on adult female Australian sea lion foraging behaviour at Liguanea (n=4), West Waldegrave (n=4), Olive I. (n=6) and Lilliput I. (n=5). Due to the difficult nature of capturing animals and inherent risk of gear loss, transmitters were not deployed at Nicholas Baudin. ). Paired whisker samples were collected from 15 mother-pup pairs at West Waldegrave, Olive I. and Liguanea I. Blood plasma samples were collected from 14 adult female Australian sea lions and their pups from West Waldegrave (n=5), Olive I. (n=4) and Lilliput I. (n=5). Stable isotope samples (pup whiskers) and DNA sampels were collected from Lounds I.(n=14), Purdie I.(34), West I. (n=14), Fenelon I. (n=5), Olive I. (n=100), Nicholas Baudin (n=50), Jones I. (n=11), West Waldegrave (n=56) and Liguanea I. (n=30)
Findings
Tracked adult female Australian sea lions could be successfully classified as either inshore or offshore foragers by mean dive depth, mean distance from shore at the inflection point of a foraging trip and whisker 13C/15N ratios. Pup isotope ratios showed a strong significant linear relationship to maternal ratios, validating the assumption that pup isotope values could be used as a proxy for maternal values.
Whilst the relationship between pup and maternal 13C was significant, there was no mean enrichment from mother to pup suggesting that no isotope discriminating factor is required to estimate maternal levels. In contrast, there was a significant enrichment of 15N in pup whiskers (1.27‰) equivalent to approximately one trophic level. Blood plasma isotope values complemented data from whiskers with a greater nitrogen discriminating factor being detected (1.92‰).
The ‘inshore’ v.s ‘offshore’ foraging ecotype dichotomy described at Lilliput, Blefuscu I. and the Pages I. in AMMC Report 0708-26 was repeated at Olive I. with offshore foraging being most prevalent. Nested within the offshore ecotype were two significantly different 15N groupings. Adult females at Jones I. expressed offshore foraging almost exclusively (90%). Nicholas Baudin and West Waldegrave were also represented by inshore (86% and 63% respectively) and offshore (14% and 37% respectively) foraging ecotypes though no significant trophic differences between ecotypes were observable. Liguanea I. was characterised by three significantly different stable carbon groups – inshore (43%), intermediate (30%) and offshore (27%). Interestingly, the offshore group were ~ 1.4‰ lower in 15N suggesting offshore-foraging animals targeting prey at a lower trophic level than inshore-foragers.
Samples from the Nuyts Archipelago (Lounds, Purdie, West and Fenelon Islands) are currently being processed and data should be available shortly.
